Insta-board

The goal is to build a big board out of nowhere using one of several summoning spells plus Darkest Hour.

Mulligan

Keep Darkest Hour, card draw. Keep at most one a summoning spell, maybe none: there are quite a few summoning spells in the deck, so if you're going to draw looking for Darkest Hour, you'll probably draw a summoning spell as well.

Replacements

All of the legendaries can be replaced by other big minions that don't rely on a battlecry.

You do need two copies of Darkest Hour; with one copy you're not going to draw it soon enough in most matches.